This invention relates to optical waveguides and more particularly to an improved process for making the preform for an optical waveguide.
Waveguides used in optical communications systems are herein referred to as "optical waveguides" and are normally constructed from a transparent dielectric material such as glass or plastic.
U.S. Pat. No. 3,775,075--Keck and Maurer describes a process of making optical waveguides wherein a glass coating is deposited by flame oxidation upon a rotating glass cylinder. This process is now called "outside vapor phase oxidation" (OVPO); the reaction is known to be oxidation rather than hydrolysis. Thereafter the formed structure is heated and drawn into a waveguide with the glass cylinder forming the core and the coated material forming the cladding of the optical waveguide. The glass cylinder upon which the cladding material is coated is referred to as a preform. Such processes are now referred to as outside vapor phase oxidation processes (OVPO) because the reaction is known to be oxidation rather than hydrolysis.
U.S. Pat. No. 3,823,995--Carpenter describes one technique of making a preform. In this technique, glass is deposited by flame outside vapor phase oxidation (OVPO) on a rotating starting member which is referred to as a bait rod. After the core and cladding layers are deposited, the bait rod is removed. Then the structure is heated and drawn. During the drawing operation, the hollow core of the preform collapses, thereby forming a consolidated core with a cladding.
The removal of the bait rod from the preform must be carefully carried out because imperfections on the inside surface of the preform may cause flaws in the finished waveguide. Imperfections on the inner surface of the preform propagate cracking and breaking. Any imperfection in the center hole may cause bubbles to form in the waveguide which is ultimately produced causing high attenuation and rejection. As described in the aforementioned Carpenter patent, the bait rod is removed by grinding it out by means of a diamond reamer. That patent also mentions core drilling and hydrofluoric acid etching for removing the bait rod. The rough surfaces left by drilling may be smoothed by laser milling, mechanically polishing the inside surface, fire polishing the inside surface and/or washing the tube in hydrofluoric acid. All of these operations are time-consuming.
More recently, the preform has been made by first depositing a soft layer of soot on the bait rod and thereafter building up the preform with hard soot. The layer of soft soot facilitates removal of the bait rod. The soft layer shears, leaving a flaky pitted center hole in the preform. After sintering the preform to consolidate the glass, the preform still contains these flaws. A lengthy hydrofluoric acid treatment is necessary for elimination of these imperfections. Damage occurs because the preform adheres so strongly to the surface of the bait rod. When the rod is removed, the preform shears rather than releasing from the bait rod surface.
U.S. Pat. No. 3,933,453--Burke et al, describes an improved mandrel including a tubular member formed of refractory metal. U.S. Pat. No. 3,806,570--Flamenbaum et al describes the use of a fused carbon mandrel for the bait rod. However, neither of these mandrels can be removed from the preform without causing blemishes which require smoothing.
Imperfections in the center hole of the preform are a particular problem in making gradient index waveguides such as described in the aforementioned Carpenter patent. In order to get a high numerical aperture waveguide, it is necessary to introduce a sharp radial change in composition of the preform. Because of the changing thermal expansion coefficient of this composition, high stress is present. In such waveguides, it is particularly important to eliminate imperfections in the center hole of the preform. Any imperfections may lead to fracture in a high stress waveguide.